Atomic Structure: Question 8

Worked solution

Step 1: Count the electrons in $\text{Fe}^{3+}$

A neutral iron atom has 26 electrons. The $\text{Fe}^{3+}$ ion has lost 3 electrons:

$\text{electrons in Fe}^{3+} = 26 - 3 = 23$

Step 2: Apply the rule for removing electrons from a transition-metal atom

Although the $4s$ sub-shell fills before $3d$ when building up the neutral atom, once electrons must be removed to form a positive ion, the $4s$ electrons are always removed first. This is because, once the $3d$ sub-shell contains electrons, its orbitals become lower in energy than $4s$, so the $4s$ electrons are the least tightly held and are lost first.

Starting from neutral iron’s configuration, $...3d^6\,4s^2$, remove the two $4s$ electrons first:

$...3d^6\,4s^0 \quad (\text{24 electrons so far})$

A third electron must still be removed to reach the $3+$ charge, and with $4s$ now empty, this comes from $3d$:

$...3d^5$

Step 3: Write the full configuration

$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^5$

Check the total: $2+2+6+2+6+5 = 23$ ✓, matching the electron count for $\text{Fe}^{3+}$. This is option A.

Step 4: Why the other options are wrong

• B ($3d^3\,4s^2$, total 23): has the correct total electron count but the wrong distribution. It removes electrons from $3d$ before $4s$, which is the reverse of the correct order for ion formation.
• C ($3d^6\,4s^1$, total 25): has removed only one electron overall, giving the configuration of $\text{Fe}^+$ rather than $\text{Fe}^{3+}$.
• D ($3d^6\,4s^2$, total 26): is simply the neutral iron atom’s configuration, with no electrons removed at all.

Final answer

• The correct configuration is $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,\boxed{3d^5}$, option A.